Melt material in general today can be processed and treated through granulation. Extruders or melt pumps are frequently used in the granulation of melt material, such as for granulation of plastics. These extruders or melt pumps press molten plastic raw material through nozzles of a perforated plate into a coolant, such as water.
In this process, the material emerging through the openings of the nozzles is cut by a cutter arrangement with at least one rotating blade to produce pellets. Corresponding devices, which carry out methods for underwater granulation, for example, are known as underwater pelletizers, for example under the product name SPHERO™ from Automatik Plastics Machinery GmbH of Germany.
Systems for carrying out hot-cut pelletization in air as the coolant have been on the market for quite a long time, since they represent relatively easy-to-build machines for pelletizing extruded thermoplastics. In these machines, strands of melt emerging from a perforated plate are chopped by blades rotating as closely as possible to the surface, and are formed into pellets by the inertia inherent in the small pieces of strand material. As a result of the rotation of the blades, air is drawn in from the environment or the interior of the housing, and the air directs the pellets more or less freely and centripetally away from the cutting location.
Typical problems in these systems relate to poor cooling of the blades, which over the course of time can overheat and stick, as well as a tendency for general sticking and clogging of the systems, especially at high throughput rates with large quantities of pellets to be produced under real world conditions.
Furthermore, pellets produced in this way tend to have cylindrical and irregular shapes, especially when the viscosity of the melt material is relatively high. In the case of pharmaceutical materials in particular, a great many pellets of uniform size and shape are more likely to be required in the downstream applications. Furthermore, pharmaceutical applications often require spherical pellets.
When using the hot-cut pelletization method, a molten polymer matrix, is pressed through an arrangement of one or more nozzles terminating in a flat surface over which passes a cutter arrangement consisting of one or more blades. The emerging strand is cut by the blade or blades into small units, called pellets, each of which is initially still molten.
Subsequently the pellets are cooled to below the solidification temperature of the polymer matrix so that they solidify. As pellets solidify, they doing lose the inherent stickiness of the melt and the tendency to adhere to surfaces or other pellets.
In accordance with the prior art, a distinction is made here between methods that use a liquid coolant, known as underwater hot die-face pelletizing, and those that do not use a liquid coolant, known as air-cooled hot die-face pelletizing. Air-cooled hot die-face pelletizing can refer to the cooling of pellets without a liquid medium, or with a mist consisting of a mixture of a gas and droplets of a liquid.
The latter group is further differentiated by the type of additional cooling method that is downstream in terms of processing, such as water ring pelletizers, in which a water film flows over the wall of the cutting chamber, which has a more or less cylindrical to truncated conical shape, for pellets to drop into and for transportation out of the cutting device.
If contact with water is undesirable for products to be granulated, pelletizers are used in which the freshly cut, still molten pellets are cooled exclusively by the cooling and transport gas. It is nonetheless typical in pelletizing machines that the freshly cut pellets are accelerated radially outward by the centrifugal force of the cutter arrangement, and also that the cooling process proceeds relatively slowly. Therefore, pellets must travel a relatively long distance in free flight before being allowed to come into contact with a surface.
As a result, such pelletizers are very large, even for low throughputs. The large size and the relatively low coolant gas flow rate results in internal turbulent flow, causing pellets to come into contact with the housing parts and other machine parts before they are cooled, where they can stick.
Moreover, ambient air is typically drawn in as the coolant gas. Ambient air can be laden with dust and undesirable substances, and often it is difficult (if not impossible) to monitor the temperature, moisture content, and freedom from dust properties.
Therefore, in order to achieve operation of a pelletizer that is as trouble-free as possible, it would be desirable for the pellets to cool sufficiently rapidly that they already have a solidified surface before they come into contact with housing or cutter parts or with other pellets.
The cooling rate is primarily a function of the temperature gradient and secondarily a function of the rapid exchange of volume elements of the gas with one another, which is referred to in the technical field as the degree of turbulence. The Reynolds number can be used as the parameter for the degree of turbulence. In this context, the cooling effect depends primarily on the properties of the polymer melt (specifically temperature, thermal capacity, surface, thermal conductivity, particle size, and specific surface), and of the coolant gas itself (specifically temperature, thermal capacity, degree of turbulence, coolant gas/polymer pellet mass flow ratio).
Most of these factors are either material constants or parameters determined by the process technology, so only a few possibilities exist for influencing the intensity of the cooling effect. In the final analysis, the heat content of the polymer pellets must be transferred to the coolant gas. If heat exchange with the housing parts and other machine parts is disregarded, the heat content difference in the melt material is equal to the heat content difference in the coolant gas.
Simple adjustability of the volume flow rate of the cooling fluid to a cutting chamber of a pelletizing device would thus be desirable for feeding of both liquid and gaseous cooling fluid, for example water or process air.
The published German unexamined patent application DE 10 2009 006 123 A1 does indeed already describe a method and a device for pelletizing thermoplastic material exhibiting flow-optimized inlet nozzle arrangements for the cutting chamber of a pelletizer, but adjustability of a slot width of an annular nozzle arrangement is not described there.
The object of the present invention is to provide a device for pelletizing melt material that overcomes the disadvantages of the prior art and that allows effective pelletizing that is flexible in application, generating uniform pellet size as well as uniform and consistent shape, in a manner that is economical and structurally simple to build, while reducing the tendency of pellets to stick.
These and other objects of the present invention are attained by the present embodiments.
The present embodiments are detailed below with reference to the listed FIGURE.